Optical transmission systems have come into widespread use primarily because of the ability of optical fibers to transmit at low cost much greater quantities of information than other comparable transmission media. Processing of such information normally requires that the information first be converted to an electronic form. It has long been realized that the costly and inefficient conversion to electronic form could be avoided by using optically nonlinear devices to perform such functions as modulation, switching, mixing, data processing and the like directly on the lightwaves. Lithium niobate is the most commonly used optical nonlinear medium, although certain organic crystalline materials have also been proposed. To date, such devices have not proved sufficiently manufacturable, reliable and efficient to displace electronic devices for most of the operations mentioned above.
The patent of Dirk et al., U.S. Pat. No. 4,859,876, granted Aug. 22, 1989, hereby incorporated herein by reference, describes a nonlinear optical element comprising a glassy polymer containing an optically nonlinear organic moiety. The nonlinearity results from electric poling during fabrication which permanently aligns dipoles within the polymer. The glassy polymer may be polymethylmethacrylate (PMMA), although other acrylate based polymers were also mentioned. The Dirk et al. patent represents a significant advance of the stage of the art since polymers such as PMMA can be applied as a thin film to substrate and their properties controlled much more easily and accurately than crystalline substances. The PMMA films constituting the hear of the various disclosed electrooptic devices may range from only about one micron to about two hundred microns in thickness.
One problem with the nonlinear optical devices proposed by Dirk et al. is that they are somewhat difficult to fabricate. It is difficult to pattern the PMMA films to define precisely the lightwave propagation paths and to electrically pole the nonlinear regions with efficiency. It is particularly difficult to define paths for single-mode propagation of light as is required for most efficient operation of optically nonlinear devices.
Accordingly, there is a long-felt need for ways to improve the manufacturability and efficiency of nonlinear optical devices.